1. Field of the Invention
The invention relates to pressure swing adsorption processing for the separation of gas mixtures. More particularly, it relates to enhanced efficiency in the use of pressure swing adsorption processing for the large scale production of oxygen from air.
2. Description of the Prior Art
Pressure swing adsorption (PSA) processes are well known for use in air or other gas separation operations. Such PSA processing generally includes a processing sequence comprising: (1) adsorption, with feed gas being passed to the feed end of an adsorbent bed at an upper adsorption pressure for the selective adsorption of a more readily adsorbable component, and with discharge of a less readily adsorbable component from the product end of the bed; (2) desorption, with depressurization of the adsorbent bed from the upper adsorption pressure to a lower desorption pressure, and with discharge of the more readily adsorbable component from the bed; (3) purging, by the passing of a purge gas to the adsorbent bed to facilitate the removal of said more readily adsorbable component from the adsorbent bed; (4) repressurization, with the pressure of the bed being increased from its lower desorption pressure to the upper adsorption pressure, and (5) passage of additional quantities of feed gas to the adsorbent at the upper adsorption pressure in step (1) as the processing sequence is continued on a cyclic basis. Such PSA processing is disclosed in the Skarstrom patent, U.S. Pat. No. 2,944,627, and a wide variety of processing variations are known in the art for the modification of the basic adsorption/depressurization/purge/repressurization sequence for various purposes.
Wagner, U.S. Pat. No. 3,430,418, discloses an adsorption system having at least four adsorbent beds wherein, as part of the desorption step in each bed, void gas, generally comprising the less readily adsorbable component, is released from the product-end of the bed and passed to the product end of another bed in the system initially at a lower pressure to equalize the pressure between the beds at an intermediate pressure level. Following such cocurrent depressurization-pressure equalization step, the bed is countercurrently depressurized from the intermediate pressure to a lower pressure with release of more readily adsorbable component from the feed end of the bed. The Doshi patent, U.S. Pat. No. 4,340,398, discloses a PSA process using three or more adsorbent beds, wherein void gas is passed from the product end of a bed, not directly to another bed, but to a storage tank from which gas is passed to a bed for repressurization purposes. Likewise, Krishnamurthy et al., U.S. Pat. No. 4,816,039, discloses the use of one or more storage tanks in a two-bed PSA system. Following direct pressure equalization between two beds, the patent discloses the passage of additional void gas from the product end of the bed being depressurized to at least one storage tank. Following regeneration of the bed at the lower desorption pressure, the void gas is returned from the tank to the bed for pressure equalization purposes. Recovery of the less readily adsorbable component product gas is enhanced due to a decrease in the loss of void space gas during subsequent countercurrent depressurization and purge steps.
In the Yamaguchi et al. patent, U.S. Pat. No. 5,258,059, a PSA process and system are described in which at least three adsorbent beds are employed, with direct bed-to-bed pressure equalization being carried out during the depressurization/repressurization portion of the processing cycle. A holding column, i.e., a segregated storage tank, of a feed-in/feed-out sequence returning type, is used for storing void space gas recovered during a cocurrent depressurization step of the cycle, with release of gas from the product end of the bed. This void space gas is then used for purging the adsorbent bed during the bed regeneration portion of the cycle. The holding column is specifically designed to prevent gas from mixing therein, i.e., an impurity concentration gradient is maintained in the holding column.
In currently used PSA cycles, the adsorbent bed undergoing a pressure equalization-pressure rising step receives product gas with decreasing purity levels from another bed currently on the make product step, i.e., the cocurrent depressurization portion of the overall make product step that includes the feed-upper adsorption pressure step and the cocurrent depressurization step. Consequently, at the end of this pressure equalization-pressure rising step, the lowest purity gas is at the product end of the bed. In addition, the gas used for purging the adsorbent bed is of decreasing purity when it is obtained from another bed currently on the make product step. If the purge gas were obtained from a product storage tank, a constant purity purge gas would be available.
It should also be noted that, in order to maintain desired product purity in prior art PSA cycles, the production and pressure equalization-falling steps must be terminated much earlier than the time required before the impurity front of more readily adsorbable component breaks through from the product end of the bed. As a result, the adsorptive capacity of the adsorbent bed is not fully utilized. Furthermore, using less readily adsorbable gas of decreasing purity during the purging, pressure equalization-rising, and repressurization steps, results in additional contamination of the product end of the bed, due to the use of the lowest purity product gas at the end of these bed refluxing steps. This added contamination of the product end of the bed results in a significant reduction in product purity in the early stage of the make product step, and causes a decrease in the average purity of the less readily adsorbable product gas. In addition, by using product gas of decreasing purity, the spreading of the mass transfer zone within the bed is undesirably enhanced. Furthermore, in order to contain the mass transfer zone and maintain product purity, more adsorbent material is required, resulting in a higher bed size factor, and a more costly overall PSA process.
In a typical prior art pressure equalization cycle, the PSA process comprises the following sequence:
(I) Feed (air) pressurization (FP) to an upper adsorption pressure level. PA1 (II) Adsorption and gross product production (AD). PA1 (III) Depressurization-Equalization falling (EQ) (cocurrent), wherein the gas is transferred to another bed that is undergoing the equalization rising step (EQ). PA1 (IV) Depressurization/Evacuation (EV) to waste (countercurrent) at a lower desorption pressure. PA1 (V) Depressurization/Evacuation to waste while purging (PG) (countercurrently). PA1 (VI) Equalization rising step (EQ), wherein the gas is supplied by another bed undergoing the equalization falling step (step III).
In another prior art product pressurization cycle, the gas required for purging and repressurization, i.e., refluxing, comes from another bed undergoing the adsorption/production step. In this mode of operation, the purge gas is obtained from another bed at an early stage of the adsorption step, with product gas being obtained from the bed during a later stage of said adsorption step. Since the effluent purity decreases with time as the impurity front of more readily adsorbable component approaches a breakthrough condition, a higher purity gas is used for purging than for product repressurization. Ideally, however, it would be desirable to use the lowest purity gas at the start of the purging step, followed by the use of product gas of increasing purity in the latter stages of such purging step. However, due to the mode of operation in such prior art PSA cycles, it is very difficult to arrange for the use of the highest purity gas last. Consequently, in order to maintain a given product purity, the percentage of the total cycle time allocated to the production of the less readily adsorbable component product gas is reduced, with a concomitant and undesired increase in bed size factor and power consumption.
In order to use the lowest purity gas at the start of the purge step, followed by product gas of increasing purity during the rest of the refluxing steps, it is necessary to produce multiple purity products, so that the highest purity gas can be used last. However, during the production step at the upper adsorption pressure, the purity of the gas removed from the product end of the bed decreases with time. Thus, the purity of the gas recovered is initially high and gradually decreases to a lower level. Thus, there is a need in the art for a means to reverse this purity order, and for the production of multiple purity products.
Since multiple purity products are required for refluxing and bed repressurization, the PSA cycle becomes inherently more complicated. In one approach to this problem, the use of two storage tanks has been considered, so that, at different times in the production step (b), the effluent gas can be directed to different storage tanks. In such a mode of operation, the time allocated for each storage tank to receive effluent gas controls the quantity of each purity gas collected. However, the use of more than one storage tank adds to the complexity and the capital cost of the PSA process, particularly since additional valves and associated piping are required thereby.
Alternatively, a single segregated storage tank can be used to store multiple purity products. In such a tank, no mixing of the product gas is allowed, and one end contains the lowest purity gas and the other end contains the highest purity gas. Such segregated storage tanks can be of the type described in the Yamaguchi et al. patent referred to above or can be a tank packed with layers of adsorbent(s) or inert materials, or simply an empty tank containing baffles to suppress mixing.
It will be appreciated from the above that there is a need in the art for the development of PSA processing improvements to enable gases of increasing purity to be used in purging at lower desorption pressure, pressure equalization-rising, and bed repressurization to the upper adsorption pressure. Such improvements would serve to lower the bed size factor and the power consumption required as compared to the requirements of prior art PSA processing cycles.
It is an object of the invention, therefore, to provide a process for using gas of increasing purity in various steps of bed regeneration to lower the bed size factor and power consumption requirements of a PSA operation.
It is another object of the invention to provide a process in which gas of increasing purity can be used throughout the purging, pressure equalization-rising and pressurization steps of a PSA cycle instead of the decreasing purity of direct bed-to-bed gas passage.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.